Delta, Nu, Omicron ( But not Xi) are dumbed down summaries. The Coronavirus has thousands of forms with some families on the uptick.

 This chart and map show how many varieties of the Covid-19 inducing virus had spurng up and been spotted in the labs in the first sixty days and where they were most common.    We're a year and a half past that.  Back then "Blue" was mostly in China and "Red" was mostly in western US and Canada.